Ferrous sintered alloy, process for producing ferrous sintered alloy and connecting rod

ABSTRACT

A ferrous sintered alloy includes a sintered raw-material powder that is made of an Fe—Cr—Mo-system powder, a carbon-system powder and an Mn—Si-system powder before sintering. The ferrous sintered alloy exhibits a density of 7.4 g/cm 3  or more, and has a metallic structure that includes martensite and bainite. In the metallic structure, the martensite accounts for an area proportion of 40% or less when the entirety of the metallic structure is taken as 100% by area. Moreover, the martensite exhibits a particle diameter of 20 μm or less. The ferrous sintered alloy is good in terms of machinability.

INCORPORATION BY REFERENCE

The present invention is based on Japanese Patent Application No. 2009-135,668, filed on Jun. 5, 2009, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ferrous sintered alloy that is made by sintering an iron-system powder, and a process for producing the same.

2. Description of the Related Art

Powder metallurgy methods have been used widely in the manufacture of mechanical component parts, because impurities are less likely to mingle in the course of powder metallurgy methods and because powder metallurgy methods make it feasible to mass-produce products that have complicated configurations with good accuracy. For examples, mechanical component parts, such as connecting rods and bearing races, are manufactured by forging heated powder compacts (or preforms for forging), or by cutting sintered bodies. A connecting rod, for instance, is manufactured by hot forging a preform for forging to a desired configuration by means of sinter forging process and then subjecting the hot-forged preform to cutting works that serve as finish processing. A ferrous sintered alloy, which is employed on this occasion, is required, not to mention, to be strengthened much further in order to provide connecting rods with higher performance and make them lightweight, but so as to make them exhibit machinability as well. However, it is very difficult to improve not only strength but also machinability, because the two properties are inconsistent with each other.

For connecting rods that are manufactured by means of sinter forging, a ferrous powder comprising an Fe-2% by mass Cu-0.6% by mass C alloy has been used generally. The addition of Cu is very effective in enhancing the strength of ferrous sintered alloy, because the circumference where Cu is present in a high concentration is likely to turn into martensite. The fatigue strength of the Fe-2% by mass Cu-0.6% by mass C alloy is improved by increasing the addition amount of copper (Cu) and reducing the addition amount of carbon (C) to the alloy. For example, Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2008-13,818 discloses a ferrous sintered alloy that comprises carbon (C) in an amount of from 0.2 to 0.4% by mass, copper (Cu) in an amount of from 3 to 5% by mass, manganese (Mn) in an amount of 0.5% by mass or less, and the balance being iron (Fe) and inevitable impurities. Moreover, it is possible to enhance the machinability of the Fe-2% by mass Cu-0.6% by mass C alloy by adding a free-cutting component to the alloy. Another ferrous sintered alloy being set forth in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2008-231,538, for instance, includes the following at least: C in an amount of from 0.4 to 1.0% by mass, molybdenum (Mo) in an amount of from 1.0 to 3.0% by mass, Cu in an amount of from 1.0 to 4.0% by mass, Mn in an amount of from 0.2 to 1.0% by mass, and sulfur (S) in an amount of from 0.05 to 0.3% by mass. S turns into a compound, manganese sulfide (MnS), and then upgrades the machinability of ferrous sintered alloy.

In addition, Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2002-20,847 discloses a connecting rod that is made of still another ferrous sintered alloy. This ferrous sintered alloy contains nickel (Ni) in an amount of from 2 to 6% by mass, Cu in an amount of from 0.5 to 2.5% by mass, Mo in an amount of from 0.5 to 1.3% by mass, C in an amount of 0.2 to 0.8% by mass, phosphorous (P) in an amount of from 0.1 to 0.3% by mass, Mn in an amount of from 0.2 to 0.65% by mass, and the balance being made up of Fe and inevitable impurities. In the ferrous sintered alloy being set forth in the publication, the coexisting Ni and Cu enhance the alloy's toughness.

As described above, it has been believed that Cu is a virtually essential additive element in ferrous sintered alloys from the viewpoint of strengthening the alloys highly. However, it is necessary to take precautions against breakage in handling preforms that are to be subjected to forging, for instance, because such preforms for forging become brittle when unsolved Cu exists in the preforms. Moreover, unsolved Cu might bring about cracks in preforms to be hot forged (i.e., hot brittleness). It is possible to inhibit cracks, which result from forging, by heating forging preforms to such a high temperature as 1,190° C. or more or making the heating time longer. However, heating them at high temperatures or for a long period of time is disadvantageous in view of manufacturing cost, because such heating degrades the production efficiency. In addition, although Cu and Ni are elements that are effective in enhancing strength, they are not only expensive but also are poor in terms of recyclability.

SUMMARY OF THE INVENTION

The present invention has been developed in view of the aforementioned problematic issues. It is therefore an object of the present invention to provide a ferrous sintered alloy, which is good in terms of machinability while making it keep exhibiting sufficient strength even when including little Cu and furthermore Ni, and a process for producing the same.

The inventors of the present invention found out that it is feasible to make ferrous sintered alloys in which strength and machinability are consistent, not by using conventional Fe—Cu—C-system powders, but by using Fe—Cr—Mo-system powders that have been regarded unsuitable for sinter forging. Then, the inventors expanded the achievement to arrive at successfully completing a variety of inventions that are described hereinafter.

That is, a ferrous sintered alloy according to the present invention, which is good in terms of machinability, comprises:

a sintered raw-material powder being made of an Fe—Cr—Mo-system powder, a carbon-system powder and an Mn—Si-system powder before sintering;

exhibits a density of 7.4 g/cm³ or more; and

has a metallic structure comprising martensite and bainite; metallic structure in which the martensite accounts for an area proportion of 40% or less when the entirety of the metallic structure is taken as 100% by area and the martensite exhibits a particle diameter of 20 μm or less.

The ferrous sintered alloy according to the present invention comprises a sintered raw-material powder. The sintered raw-material powder is made of an Fe—Cr—Mo-system powder, a carbon-system powder, and an Mn—Si-system powder before sintering. The present ferrous sintered alloy is highly strengthened because it contains C, Mn and Si in a proper amount, respectively. Consequently, it is not necessary for the present ferrous sintered alloy to include Cu that has been considered essential in order to strengthen ferrous sintered alloys. Moreover, it is possible to procure Mn and Si more inexpensively than to procure Cu comparatively. In addition, it is possible to make the present ferrous sintered alloy using Mn and Si in smaller quantities relatively. That is, the present ferrous sintered alloy also makes it feasible to reduce the raw-material cost.

Moreover, the ferrous sintered alloy according to the present invention exhibits remarkably enhanced quench hardenability, because an Mn—Si-system powder is used as an element of the raw-material powder together with an Fe—Cr—Mo-system powder. In other words, martensite structures are likely to arise in the present ferrous sintered alloy.

In addition, the ferrous sintered alloy according to the present invention exhibits a density of 7.4 g/cm³ or more. Although the present ferrous sintered alloy's density is 7.4 g/cm³ or more, the present ferrous sintered alloy not only shows high strength but also good machinability. In this specification, the term, “high strength,” means that the present ferrous sintered alloy can exhibit 960 MPa or more, or further 1,000 MPa, by tensile strength preferably.

The ferrous sintered alloy according to the present invention has a metallic structure that comprises martensite and bainite. When a ferrous sintered alloy has a metallic structure that is made of bainite independently, or when it has a composite metallic structure that is made of bainite and pearlite, the alloy is soft. Accordingly, chips, which occur while performing cutting to such an alloy, namely, a workpiece to be cut, are likely to become continuous so that adhesion might occur between a cutting tool and the workpiece. However, the present ferrous sintered alloy that is made up of martensite and bainite is hard comparatively. Consequently, chips that arise when cutting the present ferrous sintered alloy are likely to be segmented or separated from each other. Therefore, it is possible to inhibit a cutting tool and a workpiece to be cut, namely, the present ferrous sintered alloy, from adhering each other. Moreover, the greater the presence proportion of martensite a ferrous sintered alloy exhibits the harder the alloy becomes. However, ferrous sintered alloys that are hard excessively are not suitable for cutting works, because they are hard excessively so that they become brittle. On the contrarily, the present ferrous sintered alloy shows excellent machinability, because it exhibits a martensite proportion of 40% or less by area ratio.

Moreover, in the ferrous sintered alloy according to the present invention, the martensite exhibits a particle diameter of 20 μm or less. Even when a martensite proportion is kept down to 40% or less as described above, if ferrous sintered alloys, which include martensite whose particle diameter is large, are subjected to cutting work, there arises such an adverse affect that a cutting tool might be chipped or worn out during the work. When the particle diameter of martensite is set to 20 μm or less, the hardness of martensite particles themselves lowers. Therefore, it is possible to inhibit cutting tools from getting chipped and wearing off in the course of processing the present ferrous sintered alloy by cutting. In addition, when the present ferrous sintered alloy is cut, the martensite particles separate or segment the resulting chips from each other favorably because the particulate martensite exists to disperse within the alloy's matrix. As a result, the martensite particles inhibit a cutting tool and a workpiece to be cut, namely, the present ferrous sintered alloy, from adhering each other.

Furthermore, when a raw-material powder, which comprises an Fe—Mn—Si-system powder that is classified to have a particle diameter of 5 μm or less, is employed in producing the ferrous sintered alloy according to the present invention, it is possible to keep a particle diameter of the resultant martensite down to 20 μm or less in the metallic structure.

Note that the present ferrous sintered alloy involves not only simple sintered bodies but also sintered and forged bodies that are obtained by forging the simple sintered bodies. That is, in the present specification, the sintered and forged bodies as well as the simple sintered bodies are collectively referred to as “ferrous sintered alloys” regardless of being forged or not being forged in the course of their production processes.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and many of its advantages will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings and detailed specification, all of which forms a part of the disclosure.

FIG. 1 is a chart for showing the Vickers hardness of ferrous sintered alloys that were made by sintering later-described raw-material powder Nos. 1 through 30.

FIG. 2 is a chart for showing the proportions of phases that were included in the metallic structure of ferrous sintered alloys that were made by sintering later-described raw-material powder Nos. 1 through 30, and photographs that substitute for drawings for illustrating the metallic structures of the ferrous sintered alloys that were made of sintered raw-material powders Nos. 2, 12 and 22.

FIG. 3 is a graph for illustrating the relationships between a particle diameter “d” of Fe—Mn—Si-system powder and a particle diameter of martensite “D,” in the case of ferrous sintered alloys that were made by sintering later-described raw-material powder Nos. 12, 31, 34 and 35.

FIG. 4 is an explanatory diagram for illustrating a cutting test.

FIG. 5 is a graph for illustrating the wear amounts of cutting tool with respect to the number of working paths.

FIG. 6 is a photograph that substitutes for a drawing for illustrating the state of wear on the flank of chip after a cutting test was performed onto a test specimen that was made of a ferrous sintered alloy according to a comparative example, and that substitutes for another drawing for illustrating the appearance of chips that the test specimen produced when being cut.

FIG. 7 is a photograph that substitutes for a drawing for illustrating the state of wear on the flank of chip after a cutting test was performed onto a test specimen that was made of a ferrous sintered alloy according to the present invention, and that substitutes for another drawing for illustrating the appearance of chips that the test specimen produced when being cut.

FIG. 8 is a photograph that substitutes for a drawing for illustrating the state of wear on the flank of chip after a cutting test was performed onto a test specimen that was made of another ferrous sintered alloy according to the present invention, and that substitutes for another drawing for illustrating the appearance of chips that the test specimen produced when being cut.

FIG. 9 is a diagram for illustrating an automotive connecting rod schematically.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Having generally described the present invention, a further understanding can be obtained by reference to the specific preferred embodiments which are provided herein for the purpose of illustration only and not intended to limit the scope of the appended claims.

Hereinafter, preferable modes will be described, preferable modes which not only embody a ferrous sintered alloy according to the present invention and a process for producing the same but also a connecting rod comprising the present ferrous sintered alloy. Note that the numerical designations, namely, “from ‘x’ to ‘y’” as set forth in the present specification, involve the lower limit, “x,” and the upper limit, “y,” within the ranges unless otherwise specified. Moreover, it is possible to make optional numerical ranges by combining any two of numerical values that include numerical values, which are given in examples being described below, as well as the above-mentioned upper limit values and lower limit values.

Ferrous Sintered Alloy

A ferrous sintered alloy according to the present invention comprises chromium (Cr), molybdenum (Mo), silicon (Si), manganese (Mn), carbon (C), and the balance being iron (Fe) and inevitable impurities. Moreover, the present ferrous sintered alloy has a metallic structure that is made up of martensite and bainite. The martensite accounts for 40% or less with respect to the entirety of the metallic structure being taken as 100% by area. It is preferable that the martensite proportion can fall in a range of from 4 to 40%, more preferably from 4 to 25%, with respect to the entire metallic structure being taken as 100% by area. For example, the martensite proportion can be found in the following manner: a cross section of ferrous sintered alloy is observed with a microscope, and then an image analysis is carried out to calculate how much an area that results from martensite occupies or accounts for the entire area of the obtained image. The existence of martensite makes the present ferrous sintered alloy harder, and accordingly the present ferrous sintered alloy can be cut or machined satisfactorily. However, when the martensite proportion exceeds 40%, the resultant ferrous sintered alloys have become likely to embrittle because they are too hard, and have adversely showed declining machinability.

Moreover, the ferrous sintered alloy according to the present invention comprises martensite whose particle diameter is 20 μm or less. The smaller the particle diameter of martensite is the more preferable it is for the present ferrous sintered alloy. However, it is preferable that the particle diameter of martensite can fall in range of from 2 to 20 μm, more preferably from 5 to 20 μm. Note that it is not practical to make ferrous sintered alloys, which comprise a metallic structure that includes martensite having a particle diameter of less than 20 μm, from the viewpoint of production cost. The particle diameter of martensite can be a value that is obtained by observing a cross-sectional face of ferrous sintered alloy and then measuring a maximum diameter of crystalline particles in the obtained image, for instance. The “maximum diameter” herein means the maximum value of intervals between two parallel lines when the crystalline particles are held between the parallel lines.

In addition, the ferrous sintered alloy according to the present invention exhibits a density of 7.4 g/cm³ or more. The present ferrous sintered alloy's density can preferably be 7.5 g/cm³ or more. It is more preferable that the density can fall in a range of from 7.4 or more to 7.9 g/cm³ or less, much more preferably from 7.5 or more to 7.9 g/cm³ or less. The density of the present ferrous sintered alloy is set as described herein, because it is difficult to expect that ferrous sintered alloys with lower densities can be highly strengthened and their machinability can be enhanced even when the alloys include the aforementioned alloying elements and have the above-described metallic structure.

Moreover, the ferrous sintered alloy according to the present invention can preferably exhibit a Vickers hardness of from 300 Hv or more to 400 Hv or less, and can preferably exhibit a tensile strength of 960 MPa or more. Note that the Vickers hardness herein was obtained by applying a force, which fell in a range of from 5 to 20 kgf, to a unit surface area of test specimen (i.e., mm²). It is more preferable that the Vickers hardness can fall in a range of from 320 Hv to 370 Hv, and that the tensile strength can fall in a range of from 960 to 1,500 MPa, much more preferably from 1,000 to 1,210 MPa. The present ferrous sintered alloy, which is composed of alloying elements as set forth above and which comprises a metallic structure as described above, can demonstrate a Vickers hardness and tensile strength that definitely fall in the aforementioned ranges, respectively. The present ferrous sintered alloy whose Vickers hardness falls in the range of from 300 Hv to 400 Hv is much better in terms of machinability. Meanwhile, it is possible to say that the present ferrous sintered alloy exhibiting a tensile strength of 960 MPa or more, or further exhibiting 1,000 MPa or more, possesses sufficient strength as a material for applications to mechanical component parts, such as connecting rods.

The ferrous sintered alloy according to the present invention, which is made of the aforementioned metallic structure and which is further provided with the above-described physical properties, can be made by sintering a raw-material powder that comprises an Fe—Cr—Mo-system powder, a carbon-system powder and an Mn—Si-system powder. The Fe—Cr—Mo-system powder makes the major component. The carbon-system powder and Mn—Si-system powder serve as a strengthening powder, respectively.

The Fe—Cr—Mo-system powder comprises chromium (Cr), molybdenum (Mo), and the balance being Fe and inevitable impurities. Cr and Mo are elements that are effective in strengthening ferrous sintered alloy. It is suitable to set the Cr content so as to fall in a range of from 0.5 to 3.5% by mass, or further from 1 to 2% by mass, when the entire Fe—Cr—Mo-system powder is taken as 100% by mass. When the Cr content is 0.5% by mass or more, it is possible to provide the resulting ferrous sintered alloys with sufficient strength. However, the Cr content exceeding 3.5% is not preferable because the compressibility of the raw-material powder has lowered. Moreover, it is suitable to set the Mo content so as to fall in a range of from 0.1 to 2% by mass, or from 0.1 to 0.6% by mass, or further from 0.1 to 0.3% by mass, when the entire Fe—Cr—Mo-system powder is taken as 100% by mass. The Mo content being 0.1% by mass or more enables the resulting ferrous sintered alloys to exhibit sufficient strength. However, it is not preferable to set the Mo content to more than 2% not only because such an Mo content produces the effect of strength enhancement less but also because the greater Mo content results in higher material cost.

It is allowable that the Fe—Cr—Mo-system powder can further comprise silicon (Si) or manganese (Mn) as an additive element, if needed. However, excessive Si or Mn content hardens the resulting Fe—Cr—Mo-system powders so that the compactibility has lowered. Consequently, it is preferable to set the Si content to less than 0.05% by mass, or the Mn content to less than 0.2% by mass, when the entire Fe—Cr—Mo-system powder with Si or Mn further added is taken as 100% by mass.

The Mn—Si-system powder serves as a strengthening powder for enhancing the mechanical strengths of ferrous sintered alloy. The Mn—Si-system powder can preferably comprise an alloy or intermetallic compound of Mn, Si and Fe, which makes the major component of the ferrous sintered alloy according to the present invention. Such an alloy powder or intermetallic-compound powder can be procured with ease, because it can be produced inexpensively relatively by means of pulverizing an Fe—Mn—Si-system alloy ingot that has been available commercially.

It is preferable that the resulting Fe—Mn—Si-system powder can comprise Mn in an amount of from 40 to 70% by mass, Si in an amount of from 10 to 40% by mass, and the balance of Fe and inevitable impurities when the entire Fe—Mn—Si-system powder is taken as 100% by mass. Too little Mn and Si make iron alloys with ductility, and accordingly it is difficult to pulverize the resultant iron alloys to a fine powder. Moreover, too much Mn and Si are not preferable because costs have gone up due to their compositional adjustments. On the other hand, too little Mn and Si have raised the material cost of the ferrous sintered alloy according to the present invention, because it is required that an Fe—Mn—Si-system powder occupy the raw-material powder of the present ferrous sintered alloy in a greater proportion. In addition, it is more preferable that Mn and Si contents can be from 40 to 70% by mass for Mn, and from 15 to 35% by mass for Si, when the entire Fe—Mn—Si-system powder is taken as 100% by mass; and that a sum of the Mn and Si contents can fall in a range of from 75 to 85% by mass, or from 80 to 85% by mass.

Although a compositional ratio between Mn and Si in the Mn—Si-system powder does not matter, it is preferable that a compositional ratio of Mn to Si (or Mn/Si) can preferably fall in a range of from 0.5 to 4 by mass, or more preferably from 1.5 to 4 by mass. This is because it is likely to produce the present ferrous sintered alloy, which is good and well balanced in all of strength, ductility and dimensional stability, in such a preferable setting. Moreover, it is permissible that the Mn—Si-system powder can further comprise carbon (C) in an amount of from 2.5% by mass or less, or from 1.5 to 2% by mass, when the entire Mn—Si-system powder with C added is taken as 100% by mass.

The carbon-system powder introduces C into the ferrous sintered alloy according to the present invention. Although the Mn—Si-system powder strengthens the present ferrous sintered alloy, using the carbon-system powder further intends to strengthen the present ferrous sintered alloy furthermore highly. In particular, it becomes easy to improve or control the mechanical characteristics of the present ferrous sintered alloy by means of heat treatments like quenching or hardening, and tempering. Although it is feasible to employ Fe-C alloy powders and various carbide powders as the carbon-system powder, it is preferable to use a graphite powder in which C accounts for 100% virtually.

It is allowable that the raw-material powder can comprise the carbon-system powder in an amount of from 0.3 to 0.7% by mass, or from 0.5 to 0.7% by mass, and the Mn—Si-system powder in an amount of from 0.4 to 1% by mass, or from 0.5 to 0.8% by mass, when the entire raw-material powder is taken as 100% by mass. Note that the Fe—Cr—Mo-system powder makes the balance mainly. When the carbon-system powder and Mn—Si-system powder is too little, metallic structures being composed of martensite are less likely to be obtainable. The more the contents of the carbon-system powder and Mn—Si-system powder increase, the more the martensite proportion enlarges. However, too much of the carbon-system powder and Mn—Si-system powder is not preferable, because the area proportion of martensite exceeds 40%. Although the raw-material powder can preferably comprise the carbon-system powder and Mn—Si-system powder in predetermined proportions, and the Fe—Cr-Mn-system powder making the balance mainly, it is permissible that the raw-material powder can further comprise a free-cutting component, such as an MnS powder and a BN powder. It is affordable that the raw-material powder can further comprise a free-cutting component in an amount of 0.6% by mass or less, 0.3% by mass or less, or from 0.1 to 0.3% by mass, when the entire raw-material powder including the free-cutting component is taken as 100% by mass.

Meanwhile, the ferrous sintered alloy according to the present invention can be specified so as to comprise Cr in an amount of from 0.5 to 3.5% by mass, Mo in an amount of from 0.1 to 0.6% by mass, Si in an amount of from 0.04 to 0.4% by mass, Mn in an amount of from 0.1 to 0.7% by mass, C in an amount of from 0.3 to 0.9% by mass, and the balance of Fe and inevitable impurities.

Mo and Cr are elements that enhance the quench hardenability of ferrous sintered alloy. It is not possible to identify suitable contents of Mo and Cr typically, because such contents depend on addition amounts of C and the other constituent elements. However, it is preferable that the ferrous sintered alloy according to the present invention can contain Cr in an amount of from 0.5 to 3.5% by mass, more preferably from 1.3 to 1.7% by mass, and Mo in an amount of from 0.1 to 0.6% by mass, more preferably from 0.1 to 0.3% by mass, for instance, when the entire present ferrous sintered alloy is taken as 100% by mass.

Moreover, when the ferrous sintered alloy according to the present invention comprises both of Mn and Si in an appropriate amount, respectively, it exhibits greatly enhanced mechanical strength, and additionally is good in terms of dimensional stability as well.

Mn is an element that is effective in enhancing the strength of ferrous sintered alloy especially. When the entire ferrous sintered alloy according to the present invention is taken as 100% by mass, a preferable Mn content can be from 0.1 to 0.7% by mass, more preferably from 0.16 to 0.7% by mass, much more preferably from 0.2 to 0.6% by mass. Too little Mn addition produces the advantage poorly. When Mn is added excessively, not only the elongation of resulting ferrous sintered alloys decreases to decline the toughness, but also the dimension of resultant sintered products changes increasingly to disturb the dimensional-stability improvement.

Si contributes greatly to the dimensional stability of ferrous sintered alloy especially as well as to the strength enhancement of ferrous sintered alloy. In particular, when Si and Mn coexist, Si shows this tendency greatly. Mn tends to operate so as to increase the dimension of ferrous sintered alloy. On the contrary, Si tends to operate so as to decrease the dimension of ferrous sintered alloy. The coexisting two elements cancel the tendencies each other to secure the dimensional stability of ferrous sintered alloy. For example, the ferrous sintered alloy according to the present invention can preferably comprise Si in an amount of from 0.04 to 0.4% by mass, more preferably from 0.1 to 0.3% by mass, when the entire ferrous sintered alloy is taken as 100% by mass. Too little Si content is not preferable, because the resulting ferrous sintered alloys are poor in terms of the dimensional stability. Too much Si content is not preferable either, because the resultant ferrous sintered alloys show greater amount of dimensional shrinkage.

C is one of important strengthening elements for ferrous sintered alloy. Not to mention that C diffusing during sintering makes ferrous sintered alloys by means of solid-solution strengthening or hardening, but including C in an adequate amount makes it possible to subject ferrous sintered alloys to heat treatments, such as quenching and tempering. Thus, it becomes possible to enhance the mechanical characteristics of ferrous sintered alloys much more greatly. When the entire ferrous sintered alloy according to the present invention is taken as 100% by mass, a preferable C content can be from 0.3 to 0.9% by mass, or more preferably from 0.3 to 0.7% by mass. When C is added to little, the resulting ferrous sintered alloys cannot enjoy the advantages fully. An excessive C addition results in a ferrous sintered alloy with lower ductility.

Note that the ferrous sintered alloy according to the present invention can preferably make a Cu-free ferrous sintered alloy that does not include any copper (Cu) virtually, or an Ni-free ferrous sintered alloy that does not include any nickel (Ni) virtually. The present ferrous sintered alloy is of high strength without ever containing Cu or Ni. When being a Cu-free ferrous sintered alloy or an Ni-free ferrous sintered alloy that is substantially free from one of Cu and Ni, the present ferrous sintered alloy exhibits enhanced recyclability so as to become a preferable material in view of environmental measure. Moreover, it is possible to intend to produce the present ferrous sintered alloy at reduced cost, because expensive Cu and Ni can be inhibited from being made use of. In addition, when the present ferrous sintered alloy is free from Cu, it is possible to avoid the hot shortness or brittleness of ferrous sintered alloy that results from Cu. However, the present ferrous sintered alloy does not exclude the inclusion of Cu and Ni at all. That is, the present ferrous sintered alloy, which comprises Cu or Ni in an appropriate amount in addition to the above-described alloying elements, falls within the scope of the present invention. When specifying a content of Cu or Ni daringly, the content can preferably be 2% by mass or less, or more preferably 1% by mass or less, with respect to the entire present ferrous sintered alloy with Cu or Ni included being taken as 100% by mass.

Still, the ferrous sintered alloy according to the present invention constitutes a broader concept that involves ferrous-sintered-alloy members, which comprise the present ferrous sintered alloy, throughout the entire present specification. Hereinafter, some of suitable forms for producing a ferrous sintered alloy or ferrous-sintered-alloy member according to the present invention will be described.

Production Process for Ferrous Sintered Alloy

A process for producing ferrous sintered alloy according to the present invention comprises a classifying step, a raw-material-powder mixing step, a compacting step, and a sintering step. The constituent steps will be hereinafter described in detail, respectively. Note that the compositions and compounding proportions of an Fe—Cr—Mo-system powder, Mn—Si-system powder and carbon-system powder are identical with those having been indicated as above already.

(1) Classifying Step

The classifying step is a step of classifying (or sieving) the Mn—Si-system powder to particles that have a particle diameter of 5 μm or less at the maximum. Making use of the Mn—Si-system powder being sorted out to have a particle diameter of 5 μm or less makes it possible to keep a particle diameter of martensite from being more than 20 μm in the metallic structure of the resulting ferrous sintered alloy. For example, FIG. 3 illustrates a relationship between particle diameters of Mn—Si-system powder and particle diameters of martensite in ferrous sintered alloys. Note that the ferrous sintered alloys were made by sintering raw-material powders that comprised the Mn—Si-system powders with various particle diameters and a set of the Fe—Cr—Mo-system powder and carbon-system powder. In the diagram, the horizontal axis being designated as “FeMS Π's Particle Diameter “d” specifies the particle diameters of the Mn—Si-system powders, and the vertical axis being designated as “Martensite's Diameter “D”” specifies the particles diameters of martensite. As can be seen from the drawing, the resulting martensite's particle diameters are about twice as large as the Mn—Si-system powders' particle diameters. Thus, it is possible to make the resultant martensite finer reliably by employing the Mn—Si-system powder being categorized to particles with particle diameters of 5 μm or less.

Note that it is allowable to classify not only the Mn—Si-system powder but also the Fe—Cr—Mo-system powder in the classifying step. Commercially available Fe—Cr—Mo-system powders have particle diameters that are employable as the constituent element of the raw-material powder. For example, later-described “AstaloyCrL” whose maximum particle diameter is 212 μm can be used as the Fe—Cr—Mo-system powder in the raw-material powder. When making use of such a commercially available Fe—Cr—Mo-system powder, it is permissible to sort out the Fe—Cr—Mo-system powder to particles with a maximum particle diameter of 212 μm or less, 180 μm or less, or 150 μm or less. Especially, when using the Fe—Cr—Mo-system powder that is categorized to particles having a particles diameter of 150 μm or less at the maximum, it becomes likely to obtain a homogenous ferrous sintered alloy, which is less likely to suffer from componential variations or segregations, and accordingly the resulting raw-material powder makes powder compacts that exhibit enhanced powder-compact density ratios and sintered-body density ratios between the density and the theoretical density. However, it is not desirable to use the Fe—Cr—Mo-system powder that is classified to particles with 63-μm-or-less particle diameters, because the resultant raw-material powder shows lowered compactibility.

(2) Raw-Material-Powder Mixing Step

The raw-material-powder mixing step is a step of preparing a raw-material powder that comprises the Fe—Cr—Mo-system powder, the carbon-system powder, and the Mn—Si-system powder. In this step, a raw-material powder is made so that the carbon-system powder accounts for from 0.3 to 0.7% by mass, or from 0.5 to 0.7% by mass, the Mn—Si-system powder accounts for from 0.4 to 1% by mass, or from 0.5 to 0.8% by mass, and the Fe—Cr—Mo-system powder accounts for the balance, when the entire raw-material powder is taken as 100% by mass. The ferrous sintered alloy according to the present invention that comprises the above-described metallic structure is more likely to be obtainable by preparing the respective powders, which have compositions falling in the predetermined ranges as mentioned previously, and then mixing them in the predetermined compounding proportions as above. The constituent powders can be mixed to make the raw-material powder by means of ordinary methods.

Note that a lubricant agent can be further added to the raw-material powder in the raw-material-powder mixing step when the sintering step following the compacting step is a sinter forging step. That is, the Fe—Cr—Mo-system powder, the carbon-system powder, the Mn—Si-system powder and a lubricant agent are mixed in the raw-material-powder mixing step. It is allowable that a content of the lubricant agent can be 1% by mass or less, or from 0.4 to 0.8% by mass, when the raw-material powder including the lubricant agent is taken as 100% by mass. To be concrete, it is possible to name the following as the lubricant agent: zinc stearate; lithium stearate; and waxy lubricant agents, for instance, like “ACRAWAX C” (trademark) produced by LONZA JAPAN Co., Ltd., “AMIDE WAX” (trademark) produced by NIHON YUSHI Co., Ltd., and “KENOLUBE” (trademark) produced by HOEGANAES Corp. It is permissible to select one of the lubricant agents to use independently, or to select and then mix two or more of them to use.

(3) Compacting Step

The compacting step is a step of compacting the raw-material powder to turn it into a powder compact whose density is 7.4 g/cm³ or more. The clause, “density is 7.4 g/cm³ or more” indicates high-density powder compacts, which exhibit a powder-compact ratio, namely, a ratio of apparent powder-compact density to theoretical powder-compact density, that is 95% or more. It is possible to name a compacting method for powder compact that is disclosed in Japanese Patent Publication Gazette No. 3,309,970 (or U.S. Pat. No. 7,083,760), as an example that makes such high-density powder compacts producible. The compacting method will be hereinafter referred to as “die-lubricating warm pressurized compacting method” wherever appropriate. The die-lubricating warm pressurized compacting method makes it possible to perform super high-pressure compacting at industrial levels, super high-pressure compacting which is carried out at a compacting pressure of 1,000 MPa or more, 1,200 MPa or more, 1,500 MPa or more, or 2,000 MPa or more that transcends conventional levels. Thus, the die-lubricating warm pressurized compacting method results in producing powder compacts whose density can reach 96% or more, 97% or more, 98% or more, or even up to 99%.

Note that it is not needed necessarily to make the resulting powder compact's density 7.4 g/cm³ or more when a sinter forging step makes the sintering step that is carried out after the compacting step. If such is the case, it is allowable that the powder compact can exhibit a density of 7.4 g/cm³ or more when completing the sinter forging process. In other words, it is sufficient that the powder compact can have a density of 6.5 g/cm³ at least, 6.8 g/cm³ or more, or 7.0 g/cm³ or more, when completing the compacting process. Therefore, it is not required necessarily to use the aforementioned die-lubricating warm pressurized compacting method in the compacting step. That is, it is permissible to compact the raw-material powder to make a powder compact by a commonly-used method. It should be noted herein that powder compacts made by commonly-used methods can make the claimed simple powder compact (or green compact).

(4) Sintering Step

The resultant powder compact whose density is 7.4 g/cm³ or more is subjected to the sintering step. The sintering step comprises a heating sub-step, and a cooling sub-step. In the heating sub-step, the powder compact is heated. In the cooling sub-step, the powder compact, which has undergone the heating sub-step, is cooled to make a sintered body that has the metallic structure comprising martensite and bainite.

From the viewpoint of quenching, the powder compact is needed to be heated to the A₁ transformation temperature (i.e., about 730° C). or more and then to undergo austenitizing process in the heating sub-step of the sintering step. However, sintering has been usually done at a temperature of 1,050° C. or more, or 1,100° C. or more. Moreover, when intending to strengthen the resulting sintered body furthermore, a much higher sintering temperature, like 1,200° C. or more, 1,250° C. or more, 1,300° C. or more, or 1,350° C. or more, has been selected. In the process for producing ferrous sintered alloy with good machinability according to the present invention, however, the sintering step can preferably be provided with a heating sub-step in which the powder compact is heated in an inert gas atmosphere at a temperature of from 1,100° C. or more to 1,370° C. or less, or from 1,100° C. or more to 1,180° C. or less, for instance. In addition, in such a heating sub-step, the powder compact can desirably be retained in such a predetermined temperature range for a time period of from 1 minute or more, or 5 minutes or more. Still, it is allowable to set the retaining time to 60 minutes or less, or 30 minutes or less.

The cooling sub-step of the sintering step is done in succession to the aforementioned heating sub-step, and is a step of lowering the resultant ferrous sintered alloy's temperature from the sintering temperature to and around room temperature. Strictly speaking, in view of quenching, the cooling sub-step makes a step in which a temperature of the resulting ferrous sintered alloy is lowered from the sintering temperature to the M_(s) point or less. It is possible to reliably carry out quenching onto the resultant ferrous sintered alloy by means of increasing the cooling rate in the cooling sub-step. Accordingly, it is preferable to set the cooling rate to 5° C./second or more, or 10° C./second or more, for instance. In a process for producing ferrous sintered alloy with good machinability according to the present invention, however, it is possible to do quenching or hardening onto the resulting ferrous sintered alloy sufficiently even at a slower cooling rate. To be concrete, it is desirable to set the cooling rate to 30° C./minute or more, or 70° C./minute or more, though it is feasible to perform the quenching onto the resultant ferrous sintered alloy even at such a cooling rate as 100° C./minute or less. The advantage is believed to result from the fact that the ferrous sintered alloy according to the present invention shows remarkably enhanced quenchability because of the synergic effects of Cr and Mo as well as Mn and Si that the raw-material powder includes. Thus, the present invention makes it possible to quench the resultant ferrous sintered alloy without ever additionally providing such extra facilities for forcibly cooling the ferrous sintered alloy.

The process for producing ferrous sintered alloy with good machinability according to the present invention comprising the sintering step, which is provided with the above-described heating and cooling sub-steps, enables producers of ferrous sintered alloys to produce a sintered body, namely, the ferrous sintered alloy according to the present invention, which has the metallic structure that is composed of martensite and bainite as aforementioned, after completing the sintering step. That is, it is feasible to intend to reduce the costs in producing high-strength ferrous alloys, because it is possible to even complete the quenching of the resulting ferrous sintered alloy simultaneously with finishing the sintering step. Besides, not only the present ferrous-sintered-alloy production process does not require to be provided with any additional rapid-cooling facilities separately, but also can be put into practical uses on industrial scale.

Note that it is allowable that the process for producing ferrous sintered alloy with good machinability according to the present invention can further comprise a step of heat treating the resultant sintered alloy in order to control the strength or toughness after finishing the sintering step, if needed. For example, it is permissible to supplementarily subject the resulting sintered alloy to tempering, for instance, which has been carried out usually after quenching. Meanwhile, the present ferrous-sintered-alloy production process is not necessarily required to be further provided with any heat treatment, though the quenching of ferrous sintered alloys has been done commonly by doing extra heat treatments to the ferrous sintered alloys additionally. In other words, the present ferrous-sintered-alloy production process makes it possible to carry out the quenching by means of making use of the heating sub-step and subsequent cooling sub-step, which are done in the course of the sintering step.

Nevertheless, it is permissible to subject a powder compact, which comprises the raw-material powder including a lubricant agent, to another sintering step that comprises the following sub-steps: a sub-step of heating the powder compact; a sub-step of hot forging the powder compact, which has undergone the heating sub-step, to make a density of the powder compact 7.4 g/cm³ or more; and a sub-step of cooling the powder compact, which has undergone the hot-forging sub-step, to yield the sintered body.

The hot-forging sub-step alone will be hereinafter described, because the heating sub-step and cooling sub-step that make another modified sintering step are the same as those that have been described in paragraphs [0056] and [0057] above. The hot-forging sub-step is done following the heating sub-step, and is a step in which the powder compact is hot forged to make the density 7.4 g/cm³ or more. Hot forging is usually a process for forming workpiece to be processed as desirable configurations after heating the workpiece to such a high temperature as the raw material's recrystallization temperature or more. Therefore, it is allowable to subject the powder compact, which is retained at a predetermined temperature for a prescribed time in the above-described heating sub-step, to a hot-forging process as it is. As for the forging process, it is possible to name repressing (or coining), roll forging, and upset coining. The hot-forging sub-step is followed by the cooling sub-step that is done successively.

Connecting Rod Made of Ferrous Sintered Alloy

The above-described ferrous sintered alloy according to the present invention is suitable for making a whole variety of mechanical component parts. Still, the present ferrous sintered alloy is especially suitable for making connecting rods for automotive engines, because it not only exhibits high strength but also has good machinability. A connecting rod is a member for connecting a piston with a crankshaft. FIG. 9 illustrates an example of the connecting rod. As shown in the drawing, a connecting rod 70 comprises a minor end 71 at one of the opposite ends, and a major end 72 at the other one of the opposite ends. The minor end 71 is provided with a through hole 71 h into which a piston pin is fitted. The major end 72 is provided with a through hole 72 h into which a crankshaft's pin is fitted. For example, such a connecting rod can be manufactured in the following manner: forming a raw workpiece with a predetermined configuration by means of the process for producing ferrous sintered alloy with good machinability according to the present invention; and then providing the resulting raw workpiece with the through holes 71 h and 72 h, which are required in order to connect a piston with a crankshaft securely, by means of cutting work.

Although the embodiment modes of the ferrous sintered alloy, processes for producing the same and connecting rod according to the present invention have been described so far, the present invention is not limited to the above-described embodiment modes. The present invention can be conducted in various modes to which modifications and improvements are performed, modification and improvements which one of ordinary skill in the art can carry out, within a range not departing from the scope of the present invention.

Examples

Hereinafter, the present invention will be described in more detail with reference to examples of the ferrous sintered alloy, processes for producing the same and connecting rod according to the present invention.

1.1 Producing Ferrous Sintered Alloy Preparing Raw-material Powder

The following powders were prepared: an Fe-1.5Cr-0.2Mo alloy powder, and a graphite (hereinafter abbreviated to as “Gr”) powder. The Fe-1.5Cr-0.2Mo alloy powder was “AstaloyCrL” (trademark) that was produced by HOEGANAES Corp. and had particle diameters of from 20 to 212 μm. The “Gr” powder was “JCPB” (trademark) that was produced by NIHON KOKUEN Co., Ltd., and had particle diameters of 45 μm or less. Moreover, an Fe-50Mn-30Si ingot was pulverized with a vibration milling machine for 30 minutes, thereby turning the ingot into another powder alloy. The ingot was produced by NIHON DENKO Corp. The vibration milling machine was manufactured by CHUO KAKOKI Co., Ltd. These three powders were sieved to particles with particle diameters of 5 μm or less, particles with particle diameters of 10 μm or less, particles with particle diameters of 25 μm or less, and particles with particle diameters of 45 μm or less, respectively. Hereinafter, “being sieved or classified to particles with “N” μm or less” will be abbreviated to as “particle-size ‘-N’ μm.” In addition, a silicon-manganese ingot was likewise pulverized to a powder with the vibration milling machine for 30 minutes, and then the resulting powder was classified to particles with particle diameters of 5 μm or less, namely, to particle-size-5 μm particles. The silicon-manganese ingot was type #1 according to JIS (i.e., Japanese Industrial Standards). Note that Fe-50Mn-30Si alloy powder, and the silicon-manganese alloy powder will be hereinafter designated as “FeMS Π,” and “FeMSC,” respectively. Moreover, the two powders will be hereinafter referred to as “FeMS” collectively. Table 1 below shows the compositions of “FeMSΠ” and “FeMS.”

TABLE 1 Chemical Component (% by mass) Type of FeMS Fe Mn Si C O FeMS II Balance 50.2 32.8 0.06 0.42 FeMSC Balance 65.4 16.4 1.9 1.05

Note that the units for compositions are expressed in “% by mass” in the present specification. Moreover, the expression will mean the same hereinafter unless otherwise specified.

The “AstaloyCrL” powder, “Gr” powder and “FeMS” powders that had been classified as described above were weighed to make blending proportions as shown in Table 2 below. The resulting mixtures were mixed fully with a ball-milling rotary machine, thereby preparing Raw-material Powder Nos. 1 through 40 each of which comprised a uniform mixture powder.

TABLE 2 Raw-material Particle Size G.D. S.D. Δ D Hardness Powder No. of FeMS Blended Composition (g/cm³) (g/cm³) (%) Hv (at 30 kgf) 1 Not AstaloyCrL-0.5% Gr 7.61 7.57 0.19 246 Applicable 2 Not AstaloyCrL-0.6% Gr 7.59 7.56 0.20 253 Applicable 3 Not AstaloyCrL-0.7% Gr 7.57 7.54 0.22 267 Applicable 4 Not AstaloyCrL-0.8% Gr 7.55 7.51 0.23 276 Applicable 5 Not AstaloyCrL-0.9% Gr 7.54 7.50 0.23 321 Applicable 6 −5 μm AstaloyCrL-0.25% FeMS II-0.5% Gr 7.60 7.57 0.14 250 7 −5 μm AstaloyCrL-0.25% FeMS II-0.6% Gr 7.58 7.55 0.16 273 8 −5 μm AstaloyCrL-0.25% FeMS II-0.7% Gr 7.57 7.53 0.17 288 9 −5 μm AstaloyCrL-0.25% FeMS II-0.8% Gr 7.54 7.51 0.22 301 10 −5 μm AstaloyCrL-0.25% FeMS II-0.9% Gr 7.52 7.49 0.22 316 11 −5 μm AstaloyCrL-0.5% FeMS II-0.5% Gr 7.59 7.56 0.14 276 12 −5 μm AstaloyCrL-0.5% FeMS II-0.6% Gr 7.57 7.54 0.16 302 13 −5 μm AstaloyCrL-0.5% FeMS II-0.7% Gr 7.55 7.52 0.19 340 14 −5 μm AstaloyCrL-0.5% FeMS II-0.8% Gr 7.54 7.51 0.21 340 15 −5 μm AstaloyCrL-0.5% FeMS II-0.9% Gr 7.51 7.48 0.23 341 16 −5 μm AstaloyCrL-0.75% FeMS II-0.5% Gr 7.58 7.55 0.15 321 17 −5 μm AstaloyCrL-0.75% FeMS II-0.6% Gr 7.56 7.52 0.18 378 18 −5 μm AstaloyCrL-0.75% FeMS II-0.7% Gr 7.54 7.50 0.21 422 19 −5 μm AstaloyCrL-0.75% FeMS II-0.8% Gr 7.52 7.48 0.23 447 20 −5 μm AstaloyCrL-0.75% FeMS II-0.9% Gr 7.50 7.46 0.23 416 21 −5 μm AstaloyCrL-1.0% FeMS II-0.5% Gr 7.56 7.54 0.15 370 22 −5 μm AstaloyCrL-1.0% FeMS II-0.6% Gr 7.54 7.51 0.20 446 23 −5 μm AstaloyCrL-1.0% FeMS II-0.7% Gr 7.52 7.49 0.22 527 24 −5 μm AstaloyCrL-1.0% FeMS II-0.8% Gr 7.50 7.47 0.25 541 25 −5 μm AstaloyCrL-1.0% FeMS II-0.9% Gr 7.48 7.45 0.24 478 26 −5 μm AstaloyCrL-1.5% FeMS II-0.5% Gr 7.54 7.50 0.20 457 27 −5 μm AstaloyCrL-1.5% FeMS II-0.6% Gr 7.52 7.48 0.23 514 28 −5 μm AstaloyCrL-1.5% FeMS II-0.7% Gr 7.50 7.46 0.25 575 29 −5 μm AstaloyCrL-1.5% FeMS II-0.8% Gr 7.48 7.45 0.24 597 30 −5 μm AstaloyCrL-1.5% FeMS II-0.9% Gr 7.46 7.42 0.25 513 31 −45 μm  AstaloyCrL-0.5% FeMS II-0.6% Gr 7.58 7.54 0.19 297 32 −45 μm  AstaloyCrL-0.75% FeMS II-0.6% Gr 7.57 7.53 0.19 333 33 −45 μm  AstaloyCrL-1.0% FeMS II-0.6% Gr 7.55 7.51 0.19 368 34 −25 μm  AstaloyCrL-1.0% FeMS II-0.6% Gr 7.55 7.51 0.20 418 35 −10 μm  AstaloyCrL-1.0% FeMS II-0.6% Gr 7.54 7.50 0.21 469 36 −5 μm AstaloyCrL-0.5% FeMSC-0.5% Gr 7.60 7.58 0.12 288 37 −5 μm AstaloyCrL-0.5% FeMSC-0.6% Gr 7.59 7.56 0.17 313 38 −5 μm AstaloyCrL-0.5% FeMSC-0.7% Gr 7.57 7.54 0.20 332 39 −5 μm AstaloyCrL-0.5% FeMSC-0.8% Gr 7.55 7.51 0.24 392 40 −5 μm AstaloyCrL-0.5% FeMSC-0.9% Gr 7.54 7.50 0.25 376

Making Powder Compact

Making of powder compacts was carried out by means of “Die Lubrication Warm Pressurizing Compacting Method” as recited in Japanese Patent Gazette No. 3,309,970. To be concrete, a powder compact was made in the following manner. A die was made ready. The die was made of cemented carbide, and was provided with a cylindrical cavity whose diameter was φ23 mm. A TiN coating treatment had been performed onto the die's inner peripheral face in advance. The die had been heated to 150° C. preliminarily with a band heater. Then, an aqueous solution in which lithium stearate (hereinafter abbreviated to as “LiSt”) was dispersed was applied uniformly onto the inner peripheral face of the heated die with a spraying gun, thereby forming an LiSt film with about 1-μm thickness on the surface of the die's cavity. Each of the raw-material powders, which had been prepared by the above-described procedures, was filled into the cavity of the thus treated die naturally. Note that the raw-material powders had been heated to 150° C., the same temperature as that of the die, with a drier in advance. Finally, the respective raw-material powders, which were filled respectively in the die, were compacted by a compacting pressure of 1,176 MPa, thereby making powder compacts with a cylindrical configuration. Thus, each of the raw-material powders was compacted to a cylinder-shaped powder compact with a size of φ23 mm in diameter and 12 mm in length.

Making Sintered Body

The resulting powder compacts were sintered respectively in a 1,150-° C. nitrogen-gas atmosphere using a fast-cooling sintering furnace, thereby making sintered bodies (i.e., ferrous sintered alloys). The used fast-cooling sintering furnace was produced by SHIMADZU CORPORATION, and could provide variable atmospheres. Note that the powder compacts were retained at 1,150° C. for 10 minutes, and were then cooled at a cooling rate of 70° C./minute after sintering. Thereafter, tempering was carried out onto the resultant sintered bodies in air at 200° C. for 60 minutes.

Measuring Physical Properties

The powder compacts and sintered bodies, which were made in accordance with the above-described procedures, were measured for the densities. The densities were computed from the volumes of the powder compacts and sintered bodies, and their masses. The volumes were calculated from the outside diameters and heights of the cylinder-shaped powder compacts and sintered bodies that were measured using a micrometer. Moreover, the masses of the powder compacts and sintered bodies were weighed separately. Table 2 above shows the computed results. Note that the designations, “G. D.” and “S. D.” in Table 2, specify the densities of the powder compacts and the densities of the sintered bodies, respectively. In addition, the hardness of each of the sintered bodies was measured while applying a testing load of 30 kgf to the cross-sectional surface using a Vickers hardness meter. Table 2 recites the measured results, and FIG. 1 illustrates them.

Note that the content proportions of the respective elements that the sintered bodies included were substantially equal to those values that were calculated from the compositions and blended proportions of the respective raw-material powders. In some of the sintered bodies, however, their C content alone decreased slightly.

Observing Metallic Structures

A cross-sectional surface of each of the sintered bodies, which were manufactured following the above-described procedures, was observed using an optical microscope. The cross-sectional surface to be observed was prepared by subjecting each of the sintered bodies' cut cross-sectional face to an etching treatment using nital after grinding the face. An area proportion of martensite (i.e., a martensite proportion), and the particle diameter were found from an image that was obtained by the microscopic observation on the resulting cross-sectional surface of each of the sintered bodies. The martensite proportion was determined by calculating an area of martensite how much it accounted for in the entire area of the thus obtained image by means of image analysis. FIG. 2 illustrates the martensite proportions that were calculated in this manner. In FIG. 2, “M” designates martensite, “UB” designates upper bainite, and “FP” designates micro-fine pearlite, respectively. Moreover, a maximum diameter “D” of martensitic crystalline particles, and a maximum diameter “d” of FeMS Π particles were determined in the resulting image of each of the sintered bodies. Note that the maximum diameter “D” of martensitic crystalline particles herein means the maximum value of intervals being exhibited by two parallel lines that hold the martensitic crystalline particles between them. Meanwhile, the maximum diameter “d” of FeMS Π particles herein means an interval being exhibited by two parallel lines that hold an FeMS Π particle between them, FeMS Π particle which is visible at the central part of a martensitic crystalline particle that is labeled as having the maximum diameter “D” as described herein. The graph shown in FIG. 3 gives individual maximum diameters “D” and “d” of the respective martensitic crystalline particles and FeMS Π particles that were found in some of the photographed images of the sintered bodies. Moreover, the upper left of FIG. 3 schematically illustrates how to measure the maximum diameter “D” of a martensitic crystalline particle and the maximum diameter “d” of an FeMS Π particle in the martensitic crystalline particle.

Note that FIG. 2 shows not only the martensite proportions in the resulting sintered bodies but also outcomes of the microscopic observations on the cross-sectional faces of the sintered bodies that Raw-material Powder Nos. 2, 12 and 22 yielded.

1.2 Evaluation

Even using any of the raw-material powders made it possible to produce a sintered body that had a high density of 7.4 g/cm³ or more. Moreover, the obtained sintered bodies showed a dimensional change of 0.2% approximately by percentage density-change rate. In addition, the greater the added amount of “Gr” and the added amount of “FeMS” were, the higher hardness the sintered bodies exhibited.

FIG. 1 shows a relationship between Vickers hardness, varying addition amount of FeMS Π powder with particle-size-5 μm, and varying addition amount of “Gr” powder. The table in FIG. 2 gives another relationship between structural proportion, varying addition amount of “FeMS Π” powder with particle-size-5 μm, and varying addition amount of “Gr” powder. It was understood that controlling the proportions of “Gr” powder and “FeMS Π” powder that occupy in the raw-material powder makes it possible to yield a ferrous sintered alloy, which has a metallic structure that is composed of martensite and bainite, and that exhibits a martensite proportion of 40% or less by area. In particular, it was found out that a ferrous sintered alloy having such a noble metallic structure can be readily available by setting the content of “FeMS Π” powder so as to fall in a range of from 0.5 to 1% approximately, and setting the content of “Gr” powder to 0.7% or less, with respect to the entire raw-material powder. Moreover, it was ascertained that a ferrous sintered alloy being composed of the unprecedented metallic structure comes to demonstrate a Vickers hardness of from 300 to 400 Hv approximately.

It is apparent from the cross-sectional photomicrographs of the respective sintered bodies, which are incorporated into FIG. 2, that the greater the added amount of “FeMS Π” was the more the martensite proportion augmented. However, no great difference appeared between the particle diameters of martensite, because the Raw-material Powder Nos. 2, 12 and 22 for making the sintered bodies included the FeMS Π powders all of which exhibited a particle size of 5 μm or less.

FIG. 3 is a graph for illustrating a relationship between the maximum particle diameter “d” of “FeM Π” particles, which raw-material powder included, and the maximum particle diameter “D” of martensitic crystalline particles in resultant sintered body. In the graph, notice the following: the values, which lie in a range where the maximum particle diameter “d” is from 25 to 45 μm, are derived from the cross-sectional photomicrographs on the sintered bodies that were fabricated using Raw-material No. 31; the values, which lie in a range where the maximum particle diameter “d” is from 10 to 25 μm, are derived from the cross-sectional photomicrographs on the sintered bodies that were fabricated using Raw-material No. 34; the values, which lie in a range where the maximum particle diameter “d” is from 5 to 10 μm, are derived from the cross-sectional photomicrographs on the sintered bodies that were fabricated using Raw-material No. 35; and the values, which lie in a range where the maximum particle diameter “d” is from 5 μm or less, are derived from the cross-sectional photomicrographs on the sintered bodies that were fabricated using Raw-material No. 12. As can be seen from the diagram, the larger the maximum particle diameter “d” of “FeMS Π” particles was the larger the maximum particle diameter “D” of martensitic crystalline particles became. Moreover, the maximum particle diameter “D” of martensitic crystalline particles was about twice as large as the maximum particle diameter “d” of “FeMS Π” particles. In addition, employing the “FeMS Π” powder whose particle size was 5 μm or less enabled the resulting martensitic crystalline particles to exhibit a maximum particle diameter “D” of 20 μm or less. Note that measuring the Vickers hardness of martensitic crystalline particles revealed that martensitic crystalline particles whose particle diameters were 20 μm or less exhibited a hardness of from 400 to 500 Hv approximately. Meanwhile, martensitic crystalline particles having particle diameters of more than 20 μm exhibited a hardness that surpassed 500 Hv.

2.1 Producing Ferrous Sintered Alloy Preparing Raw-material Powders

In addition to the above-described “AstaloyCrL” powder, “Gr” powder and “FeMS Π” powder with particle-size-5 μm, zinc stearate (hereinafter abbreviated to as “ZnSt”), and a manganese sulfide (or MnS) powder, if needed, were weighed so that blended compositions given in Table 3 below were made ready. Note that the “ZnSt,” and the MnS powder served as a lubricant agent, and as a free cutting component, respectively. Ball-milling rotary mixing was performed fully onto the resulting mixtures to prepare six raw-material powders that were made of a uniform mixture powder, respectively. Thus, Raw-material Powder Nos. 2c, 11e through 13e, 22e and 12e⁺were made ready.

TABLE 3 Raw-material FeMS II Gr MnS ZnSt. Powder No. AstaloyCrL (%) (%) (%) (%)  2c Balance 0 0.6 0 0.6 11e Balance 0.5 0.5 0 0.6 12e Balance 0.5 0.6 0 0.6 13e Balance 0.5 0.7 0 0.6 22e Balance 1.0 0.6 0 0.6 12e⁺ Balance 0.5 0.6 0.3 0.6

Making Powder Compacts

Cylinder-shaped powder compacts were made of the above given raw-material powders with a common die-compacting apparatus. The powder compacts had a size of φ61 mm in diameter and 27 mm in thickness. Note that the compacting operation was carried out so as to make the resulting powder compacts exhibit a density of 7.0 g/cm³.

Making Sintered Bodies

The resultant powder compacts were heated in a 1,150-° C. nitrogen-gas atmosphere for 10 minutes using an ambient-heating furnace (or Erema furnace). The heated powder compacts were forged by means of coining by a facing pressure of 10 ton/cm² and were then cooled at a cooling rate of 70° C./min., thereby producing sintered forged bodies (or ferrous sintered alloys). The sintered forged bodies, which were fabricated using Raw-material Powder Nos. 2c, 11e through 13e, 22e and 12e⁺, were labeled in this order as Sintered Forged Body Nos. C2, E11 through E13, E22 and E12⁺. Note that all of these sintered forged bodies had a size of φ62 mm in diameter and 23 mm in thickness and exhibited a density that fell in a range of from 7.78 to 7.82 g/cm³. After the sintered forged products were cooled, they were subjected to tempering that was carried out at 200° C., for 60 minutes in air.

Observing Metallic Structures

The thus produced sintered forged bodies' cross-sectional face, which was prepared in the above-described manner, was observed with an optical microscope to determine how much martensite accounted for in the cross-sectional area (i.e., the martensite proportion). Note that martensite proportion was calculated in the same manner as having been described already in paragraph [0072]. Table 4 below gives the resultant martensite proportions of the sintered forged bodies.

Measuring Physical Properties

The sintered forged bodies, which were made through the above-described fabrication steps, were examined for the hardness, respectively. The hardness was measured following the method of measuring hardness that has been described already in paragraph [0070]. Moreover, each of the sintered forged bodies was tested for the tensile strength and elongation. Note that the tensile strength and elongation were found by means of procedures that conformed to “Z 2241” according to JIS (i.e., Japanese Industrial Standards).

Cutting Test

Test specimens were prepared by removing mill scales from the sintered forged bodies that were produced following the above-described procedures. The thus prepared test specimens had a size of φ61 mm in diameter and 23 mm in thickness. The test specimens were subjected to a cutting test in which a computer numerical controlled (or CNC) lathe was used. After chucking a test specimen “P” with a soft jaw 41 as illustrated in FIG. 4, an outside-diameter cutting process by turning was carried out with a cutting tool 42 for processing outside diameter. Note that the cutting tool 42 was fitted with a bit 42t. The used bit 42 t was a carbide bit “SPP-321S-CG05” produced by SUMITOMO DENKO Co., Ltd. The cutting conditions were set up as follows: the cutting speed: 195 m/min.; the feed: 0.12 mm/revolution; and the cut depth: 0.05 mm. Under the conditions, the outer circumference of the test specimens was cut by turning by 12 mm in outside diameter for every operation of the processing paths (or for every processing path), and the test specimens were subjected to the turning operation up to 150 processing paths (e.g., 30 processing paths×5 times). The finished surface of the test specimens that had undergone 30 processing paths, and the finished surface of the test specimens that had undergone 150 processing paths were measured for the 10-point average roughness Rz (as per JIS), respectively, with a surface roughness meter. Moreover, the bit 42 t was examined with a stereomicroscope for the wear amount (or wear depth) every time the test specimens had undergone processing paths. Table 4 below shows the resulting finished-surface roughness of the test specimens. FIG. 5 illustrates the resultant wear amounts of the bit 42 t.

TABLE 4 Finished-Surface Roughness Rz Martensite After 30 After 150 Sintered Proportion Hardness Tensile Strength Elongation Processing Processing Body No. (%) Hv (at 30 kgf) (MPa) (%) Paths Paths C2 0 298 952 10.1 9.3 Not Measurable E11 5 311 962 9.8 7.9 7.7 E12 10 337 1055 8.0 7.9 7.7 E13 20 362 1112 8.2 6.8 6.1 E22 40 385 1205 5.1 6.1 5.9 #12⁺ 20 344 1190 5.1 7.0 7.1

2.2 Evaluation

It should be noted that Sintered Body No. C2 was a comparative example, which was made using Raw-material Powder No. 2c that was free from “FeMS Π.” As a result, Sintered Body No. C2 did not contain any martensite in the metallic structure, and accordingly exhibited insufficient hardness and tensile strength. Consequently, Sintered Body No. C2 suffered from chipping that occurred when it underwent the processing only up to 90 processing paths only during the cutting test, and thereby could not complete the test.

On the contrarily, Sintered Body Nos. E11 through E13, E22 and E12⁺ could go through the processing of the cutting test up to 150 processing paths. For example, Sintered Body Nos. E11 through E13, E22 and E12⁺ had a metallic structure that comprised martensite and bainite, and whose martensite proportion fell in a range of from 5 to 40% by area, respectively. Moreover, Sintered Body Nos. E11 through E13, E22 and E12⁺ exhibited a Vickers hardness that came in a range of from 300 to 400 Hv; showed a tensile strength of 1,000 MPa approximately; and exhibited an elongation of from 6 to 10%. According to the results of the cutting test, Sintered Body Nos. E11 through E13, E22 and E12⁺ demonstrated a remarkable finished-surface roughness, respectively. That is, the finished-surface roughness, which they exhibited after they underwent the processing of the cutting test up to 150 processing paths, was virtually unchanged from that they showed after they experienced 30 processing paths in the course of the cutting test. Note that the values of the surface roughness, which Sintered Body Nos. E11 through E13, E22 and E12⁺ exhibited, were equivalent to that of a ferrous sintered alloy (i.e., an Fe—Cu—C—MnS material whose Vickers hardness is 270 Hv) that has been put into practical use so far. In addition, Sintered Body Nos. E12 and E12⁺ differed in that they contained the MnS powder or not. Despite the presence or absence of the MnS powder, the two sintered bodies demonstrated good characteristics regarding both of the strength and machinability.

FIGS. 6, 7 and 8 show wear states that appeared on the flank of the bit 42 t after finish cutting Sintered Body Nos. C2, E12 and E12⁺ in the cutting test, respectively, and appearances of chips that occurred during the test. As can be seen from FIG. 6, the surface of the bit 42t, which machined Sintered Body No. C2 by turning, was abraded considerably, and continuous chips occurred. On the contrary, as shown in FIGS. 7 and 8, no visible wear arose on the surface of each of the bits 42 t, which machined Sintered Body Nos. E12 and E12⁺ by turning, and the resulting chips were divided or segmented into pieces. Especially, as can been seen from FIG. 8, the surface of the bit 42 t, which machined Sintered Body No. E12⁺ by turning, was worn less, as well as the resultant chips were fractured apart favorably.

As describe above, the ferrous sintered alloys according to the present invention produced high strength of 1,000 MPa approximately, and simultaneously exhibited good machinability, which was on par with that of Fe—Cu—C material that has been made use of at present, without ever adding any free-cutting component like the MnS powder. In particular, it was understood that a raw-material powder, which comprises an Mn—Si-system powder in an amount of 0.5% by mass, a carbon-system powder in an amount of from 0.5 to 0.7% by mass, a free-cutting component, if needed, in an amount of 0.6% by mass or less, and the balance of an Fe—Cr—Mo-system powder, yields a ferrous sintered alloy that not only demonstrates high strength but also is superb in terms of machinability. Note that the compositional proportion of an Mn—Sn-system powder, which comes in a range of from 0.4 to 0.6% by mass, for instance, fall within the error range of the measurement.

Having now fully described the present invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the present invention as set forth herein including the appended claims. 

1. A ferrous sintered alloy being good in terms of machinability, the ferrous sintered alloy comprising a sintered raw-material powder being made of an Fe—Cr—Mo-system powder, a carbon-system powder and an Mn—Si-system powder before sintering; the ferrous sintered alloy exhibiting a density of 7.4 g/cm³ or more; and having a metallic structure comprising martensite and bainite; metallic structure in which the martensite accounts for an area proportion of 40% or less when the entirety of the metallic structure is taken as 100% by area and the martensite exhibits a particle diameter of 20 μm or less.
 2. The ferrous sintered alloy according to claim 1, wherein the raw-material powder comprises the carbon-system powder in an amount of from 0.3% bymass or more to 0.7% bymass or less, and the Mn—Si-system powder in an amount of from 0.4% by mass or more to 1% by mass or less when the entirety of the raw-material powder is taken as 100% by mass.
 3. The ferrous sintered alloy according to claim 2, wherein the raw-material powder comprises the carbon-system powder in an amount of from 0.5% by mass or more to 0.7% by mass or less when the entirety of the raw-material powder is taken as 100% by mass.
 4. The ferrous sintered alloy according to claim 1, wherein: the Fe—Cr—Mo-system powder comprises chromium (Cr) in an amount of from 0.5% by mass or more to 3.5% by mass or less, molybdenum (Mo) in an amount of from 0.1% by mass or more to 2% by mass or less, and the balance being Fe and inevitable impurities when the entirety of the Fe—Cr—Mo-system powder is taken as 100% by mass; and the Mn—Si-system power is made of an Fe—Mn—Si-system powder comprising manganese (Mn) in an amount of from 40% by mass or more to 70% by mass or less, silicon (Si) in an amount of from 10% by mass or more to 40% by mass or less, and the balance being Fe and inevitable impurities when the entirety of the Fe—Mn—Si-systempowder is taken as 100% by mass.
 5. The ferrous sintered alloy according to claim 1 comprising: chromium (Cr) in an amount of from 0.5% by mass or more to 3.5% by mass or less; molybdenum (Mo) in an amount of from 0.1% by mass or more to 0.6% by mass or less; silicon (Si) in an amount of from 0.04% by mass or more to 0.4% by mass or less; manganese (Mn) in an amount of from 0.1% by mass or more to 0.7% by mass or less; carbon (C) in an amount of from 0.3% by mass or more to 0.9% by mass or less; and the balance being Fe and inevitable impurities; when the entirety of the ferrous sintered alloy is taken as 100% by mass.
 6. The ferrous sintered alloy according to claim 1 being made of at least one member that is selected from the group consisting of Cu-free ferrous sintered alloys, which are free from copper (Cu) substantially, and Ni-free ferrous sintered alloys, which are free from nickel (Ni) substantially.
 7. The ferrous sintered alloy according to claim 1 exhibiting a Vickers hardness of from 300 Hv or more to 400 Hv or less; and a tensile strength of 960 MPa or more; at room temperature.
 8. The ferrous sintered alloy according to claim 1 exhibiting the martensite proportion that falls in a range of from 4 to 40% with respect to the entire metallic structure being taken as 100% by area.
 9. The ferrous sintered alloy according to claim 1, wherein the martensite exhibits a particle diameter that falls in range of from 2 or more to 20 μm or less.
 10. The ferrous sintered alloy according to claim 1 exhibiting the density that falls in a range of from 7.4 or more to 7.9 g/cm³ or less.
 11. The ferrous sintered alloy according to claim 7 exhibiting the tensile strength that falls in a range of from 960 or more to 1,500 MPa or less.
 12. The ferrous sintered alloy according to claim 1, wherein the Mn—Si-system powder comprises Mn and Si in a summed amount of from 75 to 85% by mass when the Mn—Si-system powder is taken as 100% by mass.
 13. The ferrous sintered alloy according to claim 1, wherein the Mn—Si-system powder exhibits a compositional ratio of Mn to Si falls in a range of from 0.5 to 4 by mass.
 14. The ferrous sintered alloy according claim 5 comprising Cr in an amount of from 1.3 to 1.7% by mass when the entire ferrous sintered alloy is taken as 100% by mass.
 15. The ferrous sintered alloy according claim 5 comprising Mo in an amount of from 0.1 to 0.3% by mass when the entire ferrous sintered alloy is taken as 100% by mass.
 16. A process for producing ferrous sintered alloy being good in terms of machinability, the ferrous sintered alloy having a metallic structure which comprises martensite and bainite, and in which the martensite accounts for an area proportion of 40% or less when the entirety of the metallic structure is taken as 100% by area and the martensite exhibits a particle diameter of 20 μm or less, the process comprising the steps of: classifying an Mn—Si-system powder to particles that have a particle diameter of 5 μm or less at the maximum; mixing an Fe—Cr—Mo-system powder, a carbon-system powder and the classified Mn—Si-system powder to prepare a raw-material powder; compacting the resultant raw-material powder to turn it into a powder compact whose density is 7.4 g/cm³ or more; and sintering the powder compact to prepare a sintered body that has the metallic structure, the sintering step comprising the sub-steps of: heating the powder compact; and cooling the heated powder compact to make the sintered body.
 17. The production process according to claim 16, wherein: the process is free from the compacting step, but comprises a step of compacting the raw-material powder as a simple powder compact; and the sintering step comprises the sub-steps of : heating the resulting simple powder compact; hot forging the heated simple powder compact to make a density of the simple powder compact 7.4 g/cm³ or more; and cooling the resultant hot-forged powder compact to make a sintered body that has the metallic structure.
 18. The production process according to claim 16, wherein the raw-material powder comprises the carbon-system powder in an amount of from0.3%bymass ormore to 0.7% bymass or less, andtheMn—Si-system powder in an amount of from 0.4% by mass or more to 1% by mass or less when the entirety of the raw-material powder is taken as 100% by mass.
 19. The production process according to claim 18, wherein the raw-material powder comprises the carbon-system powder in an amount of from 0.5% by mass or more to 0.7% by mass or less when the entirety of the raw-material powder is taken as 100% by mass.
 20. The production process according to claim 16, wherein: the Fe—Cr—Mo-system powder comprises chromium (Cr) in an amount of from 0.5% by mass or more to 3.5% by mass or less, molybdenum (Mo) in an amount of from 0.1% by mass or more to 2% by mass or less, and the balance being Fe and inevitable impurities when the entirety of the Fe—Cr—Mo-system powder is taken as 100% by mass; and the Mn—Si-system power is made of an Fe—Mn—Si-system powder comprising manganese (Mn) in an amount of from 40% by mass or more to 70% by mass or less, silicon (Si) in an amount of from 10% by mass or more to 40% by mass or less, and the balance being Fe and inevitable impurities when the entirety of the Fe—Mn—Si-system powder is taken as 100% by mass.
 21. The production process according to claim 16, wherein the powder compact is heated at a temperature of from 1,100 to 1,370° C. for a time period of from 1 to 60 minutes in the heating sub-step of the sintering step.
 22. The production process according to claim 16, wherein the heated powder compact is cooled at a cooling rate of from 30° C./minute or more to 100° C./minute or less in the cooling sub-step of the sintering step.
 23. The production process according to claim 16, wherein not only the Mn—Si-system powder but also the Fe—Cr—Mo-system powder are classified in the classifying step.
 24. The production process according to claim 17, wherein a lubricant agent is further added to the raw-material powder in the raw-material-powder mixing step.
 25. A connecting rod comprising the ferrous sintered alloy according to claim
 1. 26. A connecting rod being manufactured by means of the production process according to claim
 16. 